The present invention relates to the production of anthracene from a middle distillate of coal tar known as creosote.
When the condensate from the destructive distillation of coal (coal tar is distilled, a fraction is produced called "creosote" which is commonly separated into light creosote and heavy creosote, boiling between about 200.degree. C. and about 400.degree. C. Light creosote is generally considered a material having 80 weight % boiling between 235.degree. and 355.degree. C. of which 50-80 weight % boils between 315.degree. and 355.degree. C. The remaining 20 weight % of the total boils higher than 355.degree. C. Heavy creosote is generally considered a material having 50 weight % boiling between 270.degree. and 355.degree. C. and 50 weight % boiling only above 355.degree. C. While creosote is itself useful for wood preserving, several of its components are commonly first recovered because of their higher values for other uses. Anthracene is used in the production of anthraquinone. Phenanthrene and carbazole are sometimes also recovered, generally by purifying by-product streams of anthracene purification.
As described in Kirk-Othmer, Encyclopedia of Chemical Technology, Volume 1, Page 942 (1947), anthracene is conventionally produced by cooling a coal tar distillation fraction called "anthracene oil", "green oil" or "light creosote", and leaving it to stand until a precipitate forms called "anthracene salts" or "crude anthracene". The precipitate usually contains 10 to 20 percent anthracene by the method described above; but a 30-40 percent anthracene content can sometimes be obtained by modifications in the selection of a distillate fraction, by cold pressing or by simple washing.
Conventionally, anthracene of greater than about 75 percent purity (usually 90 percent purity or better) is obtained by recrystallizing or leaching crude anthracene. Common solvents for recrystallization include pyridine, furfural, tetrahydrofurfuryl alcohol, chlorobenzene, cyclohexanone, dichlorobenzene, diacetone alcohol and cresylic acid. Common solvents for leaching carbazole and phenanthrene include ammonia, as described, for example, in U.S. Pat. No. 2,011,724 to Miller, and acetone as described by an article of K. A. Belov et al. entitled "Production of Refined Anthracene from Coke-oven Crude" in 1973, Volume 8, Coke and Chemistry USSR, pages 30-33. Combinations of leaching and recrystallization are frequently employed to achieve purities of anthracene over 90 percent. Both types of purification steps produce by-product solutions containing phenanthrene and/or carbazole from which these two aromatics can also be recovered.
The conventional techniques for producing anthracene are complex and require multiple steps, first to produce the crude anthracene as a precipitate, and subsequently, to purify anthracene from crude anthracene. Furthermore, when crude anthracene of about 30 percent or higher purity is obtained in order to simplify later purification, much of the anthracene initially present in the creosote distillation fraction is not found in the crude anthracene, but instead remains from the liquid. Analyses of typical commercial operations indicate only about half of the anthracene values in the distillation fraction are recovered in the crude anthracene.